Low loss dichroic plate

ABSTRACT

A low loss dichroic plate is disclosed for passing radiation within a particular frequency band and reflecting radiation outside of that frequency band. The dichroic plate is comprised of a configuration of dipole elements defined by slots formed in a conductive plate. The slots are dimensioned so as to pass radiation of a selected frequency and are shaped so as to minimize the relationship between that frequency and the tilt angle of the plate relative to the direction of radiation. The slots are arranged so as to minimize signal power loss due to cross polarization effects.

United States Patent 1 Fletcher et al.

[ Oct. 30, 1973 LOW LOSS DICHROIC PLATE Inventors: James C. Fletcher, Administrator of the National Aeronautics and Space Administration with respect to an invention of; Richard T. Woo; Arthur C. Ludwig, both of La Canada, Calif.

Filed: Sept. 21, 1972 Appl. No.:.290,915

US. Cl. 343/909, 343/781 Int. Cl. H0lq 15/02 Field of Search; 343/755, 781, 909, 343/91 1 References Cited UNITED STATES PATENTS 9/1964 Bowman 343/909 3,165,749 l/1965 Cushner 343/909 Primary Examiner-Eli Lieberman Attorney-Monte F. Mott et al.

[57] ABSTRACT 9 Claims, 8 Drawing Figures i v 44 44c jl 5 48 5 U 44a 44b PATmTEuIIcI 30 I975 3; 769.623

SHEET 2 III 3 LQAJL JL J v JL Jill JL. U Tf FIG. 4

F I 5 MAIN POLARIZATION \J I F I G 6 2:22; CROSS POLARIZATION P TI MAIN OLARIZ ON 44c 44d I 44 48 I JK JL 5 PATENTEDBCI 30 m MAIN POLARIZATION CROSS T POLARIZATION MAIN POLARIZATION FIG; 7

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FIG. 8

LOW LOSS mcunorc PLATE The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act "6f l5ifPiiblic Law 85-568 (72 Stat. 435; 42 USC 2457). I

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to a low loss dichroic plate particularly useful in a dual frequency feed system for Cassegrainian antennas for passing polarized radiation within a selected frequency band.

2. Description of the Prior Art US. Patent Application Ser. No. 290,022, filed on Sept. 18, 1972 by Dan A. Bathker, Samuel A. Brunstein, and Arthur C. Ludwig entitled Dual Frequency Microwave Reflex Feed discloses a feed system for a Cassegrainian antenna wherein X and S band signals are simultaneously transmitted or received along a common boresight of a single antenna. That system employs a dichroic plate able to reflect signals of a first frequency (e.g., S band) and pass signals of a second frequency (X band). Dichroic plates intended for use in such systems have been discussed in the literature; e.g., (1) Ohio State University Technical Report 2148-6, 1967 and (2) IEEE Transactions on Antennas and Propegation, volume 19, No. 5, September l971 Reflection Properties of Periodic Surfaces of Loaded Dipoles. The devices disclosed in the foregoing and other papers do exhibit appropriate dichroism and have proved satisfactory in certain applications. However, the need for dichroic plates exhibiting lower power loss has been recognized for certain applications.

OBJECTS AND SUMMARY OF THE INVENTION Accordingly, an object of the present invention is the provision of a dichroic plate capable of passing a signal of selected frequency with negligible power loss.

Another object of the present invention is the provision of a dichroic plate which is essentially insensitive to the variations in the tilt angle of the plate relative to the direction of signal radiation.

A more particular object of the invention is the provision of a dichroic plate capable of passing a polarized wave form and in which dipole elements are oriented so as to cancel-cross polarization effects.

The foregoing and other objects of the invention are achieved in a preferred embodiment of the invention by providing a thin conductive plate having a matrix of slots formed therein, each slot defining a dipole element. Each slot is shaped to include an intermediate loop portion which loads the dipole element and thereby decreases the sensitivity of the plates resonant frequency to tilt angle variations. In a linearly polarized embodiment of the invention, the matrix of slots is comprised ofparallel rows in which the orientation of the slots in each row is alternated. As a consequence of the interaction between adjacent'slots, either in the same or adjacent rows, the cross'polarization effects produced by individual slots'is compensated for resulting in a nulled net cross polarization. In a circularly polarized embodiment of the invention, the plate is provided with parallel columns extending orthogonal to the rows with the slots in each column also being alternated.

FIG. 4 is a plan view of a linearly polarized dichroic plate in accordance with the present invention;

FIG. 5 is a sectional view taken substantially along the plane s s of FIG. 4;

FIG. 6 is a schematic view depicting the polarization effects occurring within the slots of the dichroic plate of FIGS. 4 and 5;

FIG. 7 is a schematic illustration of a first circularly polarized dichroic plate embodiment in accordance with the present invention; and

FIG. 8 is a schematic illustration of an alternative circularly polarized dichroic plate in embodiment accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Attention is now called to FIG. 1 which illustrates a typical system in which embodiments of the present invention can be advantageously employed. The system of FIG. 1 includes a source of X band signals 10 which radiates X band energy through a dichroic plate 12. The X band energy strikes a subreflector l4 and is reflected toward a Cassegrainian reflector 16.

A source of S band signals 18 radiates S band energy toward an ellipsoid reflector 20. The ellipsoid reflector reflects and focuses the S band energy in front of the dichroic plate 12. By correctly orienting the tilt angle of the dichroic plate, the S band signals are reflected by it in line with the X band signals toward the subreflector 14. The subreflector 14 reflects both the X and S band energy to the reflector l6 which-in turn transmits 1 both the X and S band signals along a common boresight.

The function of the dichroic plate 12 is to reflect the S band energy and to pass the X band energy with little degredation in either power or radiation pattern. These requirements necessitate that the dichroic plate have a low loss characteristic and be relatively insensitive to tilt angle, i.e., the angle of incidence of the X band energy on the plate 12. The present invention is directed to dichroic plate embodiments useful in systems of the type depicted in FIG. [for passing linearly and circularly polarized radiation of a selected frequency.

Attention is now called to FIG. 2 which schematically illustrates a known type of dichroic plate for passing linearly polarized radiation within a selected frequency band. More particularly, FIG. 2 illustrates a plurality of loaded dipole elements 30arranged in parallel rows 32, 34, 36. The dipole elements 30 may be formed, for example, by slots formed within a conductive panel such as a thin sheet of copper or aluminum. A dichroic plate of the type represented in FIG. 2 is discussed in the aforementioned paper appearing in the IEEE transactions.

Experimentation with a slotted dichroic plate of the type illustrated in FIG. 2 was conducted including the measurement of transmission loss through the plate for various tilt angles. A high transmission loss characteristic was revealed in certain orientations. In the case of perpendicular polarization, transmission loss increased with angle of incidence 6 to 2dB at 60. On the other hand, losses were less than 0.5 dB for angles of incidence up to 60 in the case of parallel polarization. The difference in behavior of these two polarizations is attributable to cross polarization effects and can be explained with reference to FIG. 3. FIG. 3 illustrates a single, short, loaded dipole element. Normally, cross polarization excited in the legs of the loop of the dipole element is equal and opposite. When the dichroic plate is tilted about the X axis in FIG. 3, corresponding to the case of parallel polarization, cross polarization in the legs of the dipole element loop remains equal and opposite and thus no net cross polarization is produced. However, when the dichroic plate is tilted about the Y axis of FIG. 3, corresponding to the case of perpendicular polarization, the net cross polarization increases with angle of incidence because of the increased phasing between the legs of the dipole element loop.

The present invention is directed to embodiments of a dichroic plate which exhibits negligible transmission loss regardless of the angle of incidence. Transmission loss is minimized in accordance with the present invention by arranging the dipole elements so that adjacent elements effectively interract to cancel cross polarization effects.

Attention is now called to FIGS. 4 and 5 which illustrate a preferred embodiment of a linearly polarized dichroic plate in accordance with the present invention. The dichroic plate of FIGS. 4 and 5 consists of a thin panel 40 of conductive material such as copper or aluminum. In a typical application, the panel 40 would be square, on the order of 32 inches along each side and have a thickness of about 0.016 inches.

A plurality of dipole elements 42 are defined in the panel 40 by forming slots 44 extending therethrough. As shown, the slots 44 are arranged in parallel rows 46, 48, 50, 52, and 54. Each of the slots 44 is illustrated as including a pair of base portions 44a and 44b and a loop portion including legs 44c and 44d.

As previously pointed out, dipole elements formed by slots with loops have been previously discussed in the literature, as for example, in the aforementioned IEEE article. The presence of the loop portion within a slot effectively electrically loads the dipole element and decreases the sensitivity of the plate s resonant frequency to tilt angle.

In order to null the net cross polarization effects discussed in connection with FIG. 3, the orientation of the slots are alternated in accordance with the present invention as shown in FIG. 4. Thus, proceeding from left to right along row 52 of FIG. 4, for example, it will be noted that the first slot has a loop pointing down, the second slot pointing up, the third slot point down, etc. In the adjacent row 54, the first slot has a loop pointing up, the next slot pointing down, etc. As a consequence of the arrangement illustrated in FIG. 4, each slot projects toward an adjacent slot as is better shown in FIG. 6. The two slots illustrated in FIG. 6 produce opposite cross polarization effects which tend to cancel or null the net cross polarization. As a consequence, a dichroic plate constructed in accordance with the teachings of FIGS. 4-6 exhibits low transmission loss. A model constructed in accordance with the arrangement illustrated in FIG. 4 was built and tested and its measured cross polarization was at least 40 dB down for angles of incidence up to 60.

It should be understood that the slots 44 formed in panel 40 should be packed with as great a density as is reasonable. The dimensions of each slot are of course, dependent upon the desired band pass frequency. More particularly, the integrated length of each slot should be approximately one half wave length. Thus, for a dichroic plate intended to pass 8.0 gigahertz radiation, the length of each slot should be approximately 0.7 inches. In the fabrication of a dichroic plate in accordance with the invention, it is important that the dimensions of the slots be uniform and thus very small tolerances, e.g., i0.00l inches, are essential. The slots 44 can be formed within the plate 40 by various fabrication techniques such as precision machining or chemical etching.

The technique employed in the linearly polarized dichroic plate of FIG. 4 to cancel cross polarization effects is also applicable to the provision of a circularly polarized dichroic plate. More particularly, attention is now called to FIG. 7 which schematically illustrates a first embodiment of a circularly polarized dichroic plate in accordance with the invention. The dichroic plate of FIG. 7 consists of parallel rows of dipole elements 60, 62, and 64 as well as parallel columns of dipole elements 66, 68, 70, and 72. It will be noted in FIG. 7 that the orientation of the successive slots is alternated along each row or column. As a consequence, cross polarization effects are cancelled as is represented in FIG. 7.

It should, of course, be recognized that arrangements of dipole elements other than those shown herein can be utilized in accordance with the present invention to achieve the net cancellation of cross polarization effects. These various arrangements are characterized by alternating the orientation of the elements along the rows and columns.

For example, one further arrangement of dipole elements in accordance with the invention is illustrated in FIG. 8 for forming a circularly polarized dichroic plate. Note that in FIG. 8 the orientation of the dipole elements is alternated along each of the rows and columns but, however, the orientation of the elements from column to column or row to row is not alternated. It is in this latter respect that the arrangement of FIG. 8 differs from the arrangement of FIG. 7.

FIG. 8 illustrates dipole rows 80, 82, 84 and dipole columns 86, 88, 90, 92. Coupling between adjacent dipole elements, as 94, 96 produces a cross polarization component to the left along column 90. However, coupling between elements 98, 100 produces an oppositely directed cross polarization component to the right along column 88. As a consequence, the net cross polarization effect is nulled.

It has been found that dichroic plates built in accordance with the teachings of FIGS. 7 and 8 exhibit very low transmission losses for angles of incidence up to 60. Even more significantly, however, such plates exhibit almost no pattern degradation which might otherwise lead to signal losses.

From the foregoing, it should now be appreciated that several embodiments of low loss dichroic plates have been disclosed herein for passing linearly or circularly polarized radiation within a selected frequency band.

What is claimed is:

l. Dichroic means for passing radiation within a selected frequency band and reflecting radiation outside of that band, said dichroic means comprising:

means defining a plurality of dipole elements each comprised of an elongated path including a first and second spaced base portions and a substantially U-shaped central portion interconnecting said first and second base portions, said elongated path having a length proportional to the wavelength at the center of said selected frequency band; and

means supporting said plurality of dipole elements in a plurality of parallel rows with the orientation of elements in each row being alternated.

2. The dichroic means of claim 1 wherein said supporting means supports some of said elements in a plurality of parallel columns interlaced with and extending orthogonal to said rows with the orientation of elements in each row being alternated.

3. The dichroic means of claim 1 wherein said means defining said dipole elements includes a thin panel of electrically conductive material having a plurality of slots extending therethrough, each slot forming said elongated path.

4. A low loss dichroic plate suitable for use in an antenna feed system for passing polarized'signals within a first frequency band and'reflecting signals outside of said first frequency band, said plate comprising:

a thin panel of electrically conductive material;

a first plurality of slots extending through said panel arranged in a plurality of parallel rows, each of said slots including first and second aligned and spaced base portions and a U-shaped central portion extending orthogonal to said base portions;

said slots in each row being arranged with the U- shaped central portions in adjacent slots extending in opposite directions.

5. The dichroic plate of claim 4 wherein each slot has alength substantially equal to one half wavelength at the center frequency of said first frequency band.

6. The dichroic plate of claim 4 wherein aligned slots in adjacent rows are oppositely oriented.

7. The dichroic plate of claim 4 including a second plurality of slots extending through said panel arranged in a plurality of columns interlaced with and extending orthogonal to said parallel rows, said slots of second plurality being shaped and dimensioned substantially identical to said slots of said first plurality and arranged along each of said columns with the U-shaped central portions in adjacent slots extending in opposite directions.

8. The dichroic plate of claim 7 wherein aligned slots ,in adjacent rows are oppositely oriented and aligned slots in adjacent columns are oppositely oriented.

9. The dichroic plate of claim 8 wherein each slot has a length substantially equal to one half wavelength at the center frequency of said first frequency band. 

1. Dichroic means for passing radiation within a selected frequency band and reflecting radiation outside of that band, said dichroic means comprising: means defining a plurality of dipole elements each comprised of an elongated path including a first and second spaced base portions and a substantially U-shaped central portion interconnecting said first and second base portions, said elongated path having a length proportional to the wavelength at the center of said selected frequency band; and means supporting said plurality of dipole elements in a plurality of parallel rows with the orientation of elements in each row being alternated.
 2. The dichroic means of claim 1 wherein said supporting means supports some of said elements in a plurality of parallel columns interlaced with and extending orthogonal to said rows with the orientatioN of elements in each row being alternated.
 3. The dichroic means of claim 1 wherein said means defining said dipole elements includes a thin panel of electrically conductive material having a plurality of slots extending therethrough, each slot forming said elongated path.
 4. A low loss dichroic plate suitable for use in an antenna feed system for passing polarized signals within a first frequency band and reflecting signals outside of said first frequency band, said plate comprising: a thin panel of electrically conductive material; a first plurality of slots extending through said panel arranged in a plurality of parallel rows, each of said slots including first and second aligned and spaced base portions and a U-shaped central portion extending orthogonal to said base portions; said slots in each row being arranged with the U-shaped central portions in adjacent slots extending in opposite directions.
 5. The dichroic plate of claim 4 wherein each slot has a length substantially equal to one half wavelength at the center frequency of said first frequency band.
 6. The dichroic plate of claim 4 wherein aligned slots in adjacent rows are oppositely oriented.
 7. The dichroic plate of claim 4 including a second plurality of slots extending through said panel arranged in a plurality of columns interlaced with and extending orthogonal to said parallel rows, said slots of second plurality being shaped and dimensioned substantially identical to said slots of said first plurality and arranged along each of said columns with the U-shaped central portions in adjacent slots extending in opposite directions.
 8. The dichroic plate of claim 7 wherein aligned slots in adjacent rows are oppositely oriented and aligned slots in adjacent columns are oppositely oriented.
 9. The dichroic plate of claim 8 wherein each slot has a length substantially equal to one half wavelength at the center frequency of said first frequency band. 